The present invention relates to a present position estimation method, a positioning method, a program, and a mobile terminal.
The global positioning system (GPS) is widely known as a positioning system utilizing a satellite, and is used for a mobile terminal such as a portable telephone. A GPS terminal locates its present position by calculating four parameters (i.e., three-dimensional coordinate value which indicates the position of the terminal and a clock error) based on information such as the positions of a plurality of GPS satellites and the pseudo-range between each GPS satellite and the terminal (i.e., positioning calculations).
In positioning calculations, convergent calculations based on the Newton's method or the like are performed on simultaneous equations relating to the pseudo-ranges of a plurality of GPS satellites to calculate a solution (located position). In this case, if the initial position is not set appropriately, the solution may converge incorrectly. Various technologies have been proposed for estimating the present position of a mobile terminal used as the initial position (e.g., JP-A-2006-71460).
These related-art technologies have been developed on the assumption that the mobile terminal moves in a period between the preceding positioning and the present positioning. In the technology disclosed in JP-A-2006-71460, the moving distance of the mobile terminal in a period between the preceding positioning and the present positioning is estimated by multiplying the elapsed time from the preceding positioning to the present positioning by the moving velocity (e.g., 100 km/h) of the mobile terminal set in advance, and the present position of the mobile terminal is determined based on the estimated moving distance and position information relating nearby base station and used as the initial position.
When the mobile terminal has not moved in a period between the preceding positioning and the present positioning, the actual moving distance of the mobile terminal is zero. However, since a large value is calculated as the estimated moving distance as the stop time becomes longer, the present position is not estimated correctly, whereby a position significantly differing from the actual position of the mobile terminal may be set as the initial position.
For example, in the case of an electronic instrument which has a positioning function and of which execution/suspension of positioning is controlled in synchronization with power ON/OFF, the position where the electronic instrument has been turned OFF may be the same as the position where the electronic instrument is turned ON when subsequently used. In this case, since a considerable period of time has expired from the positioning time immediately before the electronic instrument is turned OFF to the time when the electronic instrument is again turned ON, it is difficult to appropriately estimate the present position by applying the technology disclosed in JP-A-2006-71460.